Abstract
Insurmountable antagonism (maximal response to the agonist depressed) can result from a temporal inequilibrium involving a slow offset orthosteric antagonist or be the result of an allosteric modulation of the receptor. The former mechanism is operative when the antagonist, agonist, and receptors cannot come to proper equilibrium during the time allotted for collection of agonist response (hemi-equilibrium conditions). Allosteric effects (changes in the conformation of the receptor through binding of the allosteric modulator to a separate site) can preclude the agonist-induced production of response, leading to depression of maximal responses. In these cases, the effects on receptor affinity can be observed as well. The first premise of this article is that system-independent estimates of insurmountable antagonist potency can be made with no prior knowledge of molecular mechanism through the use of pA2 (–log molar concentration of antagonist producing a 2-fold shift of the concentration response curve) measurements The relationship between the pA2 and antagonist pKB (–log equilibrium dissociation constant of the antagonist-receptor complex) is described; the former is an extremely close approximation of the latter in most cases. The second premise is that specially designed experiments are required to differentiate orthosteric versus allosteric mechanisms; simply fitting of data to orthosteric or allosteric theoretical models can lead to ambiguous results. A strategy to determine whether the observed antagonism is orthosteric (agonist and antagonist competing for the same binding site on the receptor) or allosteric in nature is described that involves the detection of the hallmarks of allosteric response, namely saturation and probe dependence of effect.
Footnotes
-
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
-
doi:10.1124/jpet.106.107375.
-
ABBREVIATIONS: RANTES, regulated on activation, normal T cell expressed and secreted (standard nomenclature for this chemokine is CCL5); FLIPR, fluorometric imaging plate reader; Darifenacin, (S)-2-{1-[2-(2,3-dihydrobenzofuran-5-yl)ethyl]-3-pyrrolidinyl}-2,2-diphenylacetamide hydrobromide; SR 48,968, (S)-N-methyl-N[4-(4-acetylamino-4-piperidino)-2-(3,4-dichlorophenyl) butyl]benzamide; FR173657, (E)-3-(6-acetamido-3-pyridyl)-N-[N-[2,4-dichloro-3-[(2-methyl-8-quinolinyl) oxymethyl] phenyl]-N-methylaminocarbonylmethyl] acrylamide; FR190997, 8-[2,6-dichloro-3-[N-[(E)-4-(N-methylcarbamoyl)cinnamidoacetyl]-N-methylamino]-benzyloxy]-2-methyl-4-(2-pyridylmethoxy)quinoline; SB203220, [E-a-[[2-butyl-1-(carboxy-1-naphalenyl)methyl-1H-imidazol-5-yl]-methylene]-2-thiophene-propanic acid]; AMD3100, 1,1′-[phenylenebis(methylene)]bis-1,4,8, 11-tetraazacyclotetradecane octahydrochloride; aplaviroc, 4-{[4-({(3R)-1-butyl-3-[(R)-cyclohexyl(hydroxy)methyl]-2,5-dioxo-1,4,9-triazaspiro-[5.5]undec-9-yl}methyl)phenyl]oxy}benzoic acid hydrochloride; Sch-C, (Z)-(4-bromophenyl){1′-[(2,4-dimethyl-1-oxido-3-pyridinyl)carbonyl]-4′-methyl-1,4′-bipiperidin-4-yl}methanone O-ethyloxime; vicriviroc, 4,6-dimethyl-5-{[4-methyl-4-((3S)-3-methyl-4-{(1R)-2-(methyloxy)-1-[4-(trifluoromethyl)phenyl]ethyl}-1-piperazinyl)-1-piperidinyl]carbonyl}pyrimidine; maraviroc, 4,4-difluoro-N-((1S)-3-{(3-endo)-3-[3-methyl-5-(1-methylethyl)-4H-1,2,4-triazol-4-yl]-8-azabicyclo[3.2.1]oct-8-yl}-1-phenylpropyl)cyclohexanecarboxamide; TAK779, N,N-dimethyl-N-[4-[[[2-(4-methylphenyl)-6,7-dihydro-5H-benzocyclohepten-8-yl]carbonyl]amino]benzyl]tetrahydro-2H-pyran-4-aminium chloride; DR, dose ratio; pilocarpine, (3S,4R)-3-ethyl-4-[(3-methylimidazol-4-yl) methyl]oxolan-2-one; arecoline, methyl 1,2,5,6-tetrahydro-1-methyl-3-pyridinecarboxylate; HIV, human immunodeficiency virus.
- Received May 4, 2006.
- Accepted July 19, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
JPET articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|